1. Field of the Invention
This invention relates to an improved process for the production of a purified n-butanol by the hydrogenation of n-butyraldehyde and the fractional distillation of the resulting crude n-butanol.
2. Background Information Including a Description of Related Art
It is known to produce n-butanol by the hydrogenation of the n-butyraldehyde obtained, for example, by the hydroformylation of propylene by reaction with carbon monoxide and hydrogen. However, in order to be suitable for various applications, e.g., as a solvent for fats, waxes and resins, and in the manufacture of rayon, detergents and various butyl compounds, the n-butanol must have a high degree of purity including a specified low level of various impurities produced by the hydroformylation and hydrogenation reactions. To deal with this problem, the crude n-butanol produced by the hydrogenation reaction must be purified, generally by fractional distillation. One of the impurities intended to be removed by the distillation is di-n-butylether (DBE) which has an atmospheric boiling point of 142.degree. C., but in the absence of water forms a binary azeotrope with n-butanol having an atmospheric boiling point of about 117.6.degree. C. This is very close to the boiling point of pure n-butanol of about 117.2.degree. C., making it difficult to separate DBE from n-butanol when water is not present. However in the presence of water, a ternary azeotrope of water, n-butanol and DBE is formed having a boiling point of about 90.6.degree. C. which can be exploited in the separation of DBE from the bulk of the n-butanol product. Other impurities produced during the catalytic hydrogenation of n-butyraldehyde to form n-butanol are so-called "heavy ends," which are relatively high boiling and tend to break down during purification by distillation to form "permanganate time consumers" (PTC's), i.e., certain unsaturated compounds and chromophores such as olefins, aldehydes and ketones, during distillation. The PTC's, like other impurities such as DBE, may also produce an adverse effect on end use applications if present in the n-butanol product, and like DBE, their separation from the n-butanol product is facilitated by the presence of water in the distillation column.
When any of certain catalysts such as Raney nickel is used for the hydrogenation of n-butyraldehyde to n-butanol, a fairly large amount of water, e.g., about 8-15 weight percent (wt %), is added to the aldehyde feed both to decrease the amount of DBE otherwise formed as a side reaction of the hydrogenation of aldehyde in the absence of such amount of water, and to ensure that the bulk of the DBE which does form can be separated from the n-butanol product as the ternary azeotrope discussed previously, and that the removal of PTC's which form from the heavy ends in the fractionating column is also facilitated. However, the presence of such a large amount of water in the fractionating column results in a substantial expenditure of energy, generally through steam consumption, to vaporize the water present, and may also necessitate a larger column than would otherwise be necessary to carry out the purification. Thus, any change in the process is desirable which results in a decreased amount of water necessary in the reactor and the fractionating column and thus a reduction in energy consumption and possibly the size of the column, without any increase in the amount of DBE and PTC's present in the product.
The following prior art references may be considered material to the claimed invention.
U.S. Pat. No. 4,263,449, issued Apr. 21, 1981 to Saito et al., discloses a process for producing alcohols, e.g., butanol, by the hydroformylation of an alkenyl compound, e.g., propylene, and the hydrogenation of the resulting aldehyde in the presence of a hydrogenation catalyst, e.g., Raney cobalt. Water is added at a ratio of 0.5 to 30 times by weight based on the aldehyde produced by the hydroformylation before the hydrogenation.
U.S. Pat. No. 4,826,799, issued May 2, 1989 to Cheng et al., teach a process of making catalysts by the Raney process including the steps of pelletizing a Raney process metal alloy, e.g., of cobalt and aluminum, in a matrix of polymer and plasticizer followed by removal of plasticizer or plasticizer and polymer, and the leaching out of the aluminum with caustic. The catalyst may be used to hydrogenate an aldehyde to the corresponding alkanol, e.g., butanol.